The objects of the present invention are to generate interferometric signals more accurately and more precisely, and in some cases, more rapidly than is possible with present art. Accordingly a new class of tilt-compensated interferometer designs for generating interferometric signals is disclosed.
The subject area of the invention is tilt-compensation of multiple reflecting surfaces. A recently-approved application, Ser. No. 08/959,030 which disclosed optics for tilt-compensation of the moving mirror of an interferometric spectrometer, is included by reference for the entirety of its disclosure. The tilt-compensation was effected by the use the use of two complementary reflections at a flat moving mirror. The present disclosure expands the use of this tilt-compensation mechanism to a larger class of interferometers in which the compensated complementary reflections occur at one or more planar surfaces which may include the beamsplitter. A variety of motions may be applied to the moving planar surfaces to introduce path difference scanning. In conventional Michelson interferometers, tilt errors of the planar mirrors compromise photometric accuracy and interferometric efficiency. Baseline errors will also be introduced into spectra measured with instruments having tilt errors. Considerable effort has been expended in constructing interferometers which have electronic servomechanisms to adjust the tilt of planar interferometer mirrors. Considerable effort has also been expended in constructing interferometers having intrinsic optical tilt-compensation. The present invention expands the area of intrinsic optical tilt-compensation by applying a novel approach to tilt-compensate moving planar mirrors and applying a known approach for tilt-compensating beamsplitter. Thus, the present invention allows construction of interferometers that may be permanently aligned and more stable than known ways. A variety of applications will benefit from these improvements.
The tilt-compensation approach for the beamsplitter is known in, for example, Schindler, U.S. Pat. Nos. 3,809,481, 4,181,440, 4,193,693, Frosch, U.S. Pat. No. 4,278,351 and Woodruff, U.S. Pat. No. 4,391,525. The primary moving mirrors are retroreflectors and the planar mirrors were generally fixed. In the cases where a planar mirror did move, it was for correcting path difference errors introduced by imperfections in the motion of the retroreflectors. The planar reflectors make these interferometers more compact by folding the beams. Reference is also made to Solomon, U.S. Pat. No. 5,675,412 and Turner and Mould, U.S. Pat. No. 5,808,739 as well as a commercial product from Bomem (450, avenue St-Jean-Baptiste, Quebec, Quebec, G2E 5S5, Canada), the MB-100 Fourier spectrometer. The Bomem instrument uses beamsplitter, as is also shown in, for example Learner, U.S. Pat. No. 4,779,983 and Izumi, U.S. Pat. No. 4,932,780.
Tilt compensation by complementary reflections is shown in FIG. 1. A primary beam of radiation from a collimated source 10 propagates to a beamsplitter 30. The beamsplitter 30 may have a coating 32 intended to be partially reflective and partially transmitting. The primary radiation beam divided at the beamsplitter coating 32 propagates in two directions. The first energy beam is reflected by coating 32 and enters a first optical path. The second energy beam is transmitted by coating 32 and enters a second optical path. The term arm may be used interchangably with first or second optical path.
The reflected first energy beam, in the case of FIG. 1, propagates to a retroreflector 70 which returns the beam with an offset, but with a propagation angle exactly antiparallel to the incident beam. The returned first energy beam propagates to the beamsplitter 30 where it may impinge on a reflective coating 34. The beam then makes a second reflection from the beamsplitter at 34 and propagates to a fixed reflector 80 which may be flat. The first energy beam propagating towards 80 is necessarily parallel to the primary energy beam to the extent that the beamsplitter coatings 32 and 34 are exactly parallel and to the extent that the retroreflector 70 is optically perfect. In practice, these conditions can be met with sufficient accuracy for useful interferometric measurements. If the reflector 80 is oriented perpendicularly to the primary energy beam, the reflection which occurs for the first energy beam will be at exactly normal incidence causing this beam to exactly reverse its course through the first optical path where it will reach the coating 32 and recombine with a portion of the second energy beam which has traversed the second optical path. FIG. 1 only indicates such a moving mirror in the second optical path. It will be shown that one or more moving planar reflectors may be included in either or both of the first and second optical paths.
The second energy beam initially transmitted through the coating 32 impinges on a movable flat mirror 50 then propagates to a retroreflector 60. The retroreflector 60 returns the second energy beam exactly parallel and inverted. The inverted beam may then impinge a second time on the planar surface of 50 and then propagate to return reflector 80. The beam may pass through an uncoated portion of the substrate 30, or a compensator plate according to Woodruff, in transit from mirror 50 to reflector 80 and vice versa. The second energy beam as it propagates to the return reflector 80 is necessarily perpendicular to the primary energy beam to the extent of optical perfection of the components. As before, to the extent that reflector 80 is aligned perpendicular to the primary energy beam from the source 10, then the impingement of the second energy beam on 80 will be at exactly normal incidence. This completes one half of the traversal of the second optical path. Because of the perpendicular incidence, reflector 80 returns the second energy beam exactly on the inverse of the first half of its traversal of the second optical path, thus returning it to the beamsplitter 30 with optical precision. The four reflections at the mirror 50 are pairwise complementary such that the beam returning to the beamsplitter 30 via the second optical path is exactly antiparallel to the second energy beam initially entering the second optical path from the beamsplitter 30.
The two reflections of the first energy beam from the beamsplitter at coatings 32 and 34 are complementary. Hence, the beam propagating from the reflective coating 34 to reflector 80 is exactly parallel to the primary beam propagating from the source 10 to the beamsplitter 30 and its coating 32. Likewise, the beam propagating from retroreflector 60 to reflector 80, which may pass through a compensator plate, or an uncoated portion of the substrate 30 or pass around substrate 30, will be exactly parallel to the beam propagating from beamsplitter 30 and its coating 34 to reflector 80. At reflector 80, the first and second energy beams reverse their direction of propagation and return to the beamsplitter 30 by the exact inverse of their paths from it. At the beamsplitter 30, particularly coating 32, the returning first and second energy beams are both split again and form two recombined beams at coating 34. One of the recombined radiation beams returns to the source 10 and is effectively lost. The other recombined energy beam propagates to a detector 20 by a path which may include other optics and/or material to be measured as is commonly practiced in the use of interferometers.
Tilt of reflector 80 will produce only second order misalignment (motion of the source 10 image on the detector 20) because the optical alignment of the wavefronts will be preserved by the equal effect of the tilt of reflector 80 on both the first and second energy beams. It will be appreciated that the misalignment of any component in FIG. 1 will have at most second order effect. The path difference between the first and second optical paths of the interferometer thus formed may be varied or scanned by moving reflectors 50, 60 or 70, or any combination of these. Motion of reflector 80 does not usefully introduce path difference between the two arms of the interferometer, i.e., between the first and second optical paths, because it affects both equally. This optical arrangement improves on known systems. The present invention uses tilt compensation for both the beamsplitter and the moving mirrors of a class of related interferometers.
All of these objectives, features and advantages of the present invention, and more, are illustrated below in the drawings and detailed description that follows.